This invention relates generally to apparatus for reproducing originals or images, and more particularly to such apparatus utilizing a laser beam and exhibiting superior performance in the processing of the image signal.
Systems for image reproduction have recently been developed which scan an original (transparent or reflection originals of photographs, line drawings, characters, pictures, etc.) and reproduce another image, using a laser beam. Because they are capable of highspeed, high resolution, and noiseless recording and processing, such apparatus have been used as scanners, laser printers, and other equipment in the fields of printing and copying. However, there are some difficulties in the constitution of the laser optics as described below.
In the case where an apparatus of this type has rotating polygonal mirrors used as deflectors of the laser beam, in order to accurately deflect the laser beam and scan the originals, it is necessary to use polygonal mirrors having such high precision that the accuracy of the pyramidal error is within a few seconds. To obtain polygonal mirrors having such high accuracy, high-level manufacturing technology is required. In addition, because the rotation of the mirrors is performed at high speed, high-level bearing technology is also needed.
In addition, when the beam is deflected by the rotating polygonal mirrors onto the original, or recording material, if a conventional lens is used, the scanning speed on the original will be different toward the periphery from the center area, and this non-uniform scanning speed will cause distortions to appear in the image. Thus an f0 lens is used in order to correct these distortions. The f0 lens is a special large-diameter lens, and high-level technology is required for both the design and the manufacture of the lens, thus making the lens quite expensive.
In place of this special optical system as described above, mirrors, such as galvano mirrors which deflect through an angle in accordance with an electrical signal, can be used to deflect the laser beam.
FIG. 4 shows a representative example of an apparatus for image reproduction which uses a laser beam and galvano mirrors. A description of the basic construction of this apparatus is as follows. The original 1 to be reproduced is placed on the original carriage 2. The beam from a laser light source 3 is passed through a beam splitter 4. One part A of the split beam is guided on the beam path indicated by the solid lines, passed through a beam expander 5 as needed, deflected in the x direction by the galvano mirror 6, focused by the scanning lens 7, reflected by the reflecting mirror 10, and passed through the half mirror 11 in order to scan the original image 1. An amount of light corresponding to the density of the original image 1 is converted into an electric signal by a photoelectric transducer 12 including an array of photodiodes, etc., and thus becomes the image signal. This image signal undergoes amplification, analog to digital (A/D) conversion, and other processing, and then further image processing such as tone correction, image enhancement etc., is performed as needed in order to obtain a processed image signal which corresponds to the image.
Scanning of the original in the y direction is performed by driving the carriage 2 using the drive motor 13.
Meanwhile, in order to record the image, the other part B of the laser beam split by the beam splitter 4 is used. This beam B is guided as indicated by the broken line beam path shown in FIG. 4. After the beam is modulated by an optic modulator 14, via a galvano mirror 6, the surface of the recording material 15 is scanned and exposed in the x direction. The recording material 15 is moved in the y direction by a drive motor 16 via a capstan 17.
Swinging the galvano mirrors 6 and 6' at high speed in accordance with a sawtooth wave and the like would be extremely difficult. The preferred way to drive the galvano mirrors is to deflect the reflecting mirrors by resonance in a sine wave, thus making it possible to achieve a stable drive at high speed. However, with this deflection method, because the scanning speed differs between the center (in the x direction of scanning) and the periphery, the original or the reproduced image will be read or reproduced in a distorted form.
In other words, whether rotating polygonal mirrors or galvano mirrors are used as the deflection mirrors, and also no matter what design is used for the scanning lens, it is extremely difficult to completely eliminate this kind of distortion. Furtheremore, because the deflection means used in these devices requires mechanical rotation, it is impossible to obtain completely identical scanning conditions in all rotations, which means that the distorted condition just described will change slightly from one scanning to the next.
For these reasons, in addition to the laser beam used for image recording (or image reading), a laser beam is also used as a synchronizing signal, thus providing a way to correct the distortion in recording or reading due to the changes of the scanning speed. In the apparatus shown in FIG. 4, the laser beam C which is reflected by the half mirror 11 from beam A and which passes through the reflecting mirror 10 is selectively passed by a grid plate 18 containing equally spaced light shield slits, and detected by the laser beam detector 19 which forms a pulse signal S (refer to FIG. 6) having a frequency proportional to the deflection speed of the laser beam.
This pulse signal S is multiplied before use. In other words, as shown in FIG. 5, the pulse signal S from the laser beam detector 19 is fed to one input terminal of a phase comparator PC whose output is converted into a voltage V (refer to FIG. 6) by a low-pass filter LF. This voltage is supplied as the frequency control voltage for a voltage-controlled oscillator VCO (FIG. 5), and the resulting output is fed to a divider counter D, the output of which is fed to another input terminal of the phase comparator PC. The multiplication ratio of this PLL frequency multiplier circuit is equivalent to the division ratio of the divider counter D.
FIG. 6 shows the various signals discussed above of the speed distortion correction circuit shown in FIG. 5. Although the galvano mirrors (6 and 6') are driven in approximately the shape of a sawtooth wave, the incompleteness, etc., of the drive at the peripheral areas causes the scanning speed to be non-uniform. Because the light shield slits are equally spaced in the grid plate 18, a pulse signal S having a frequency proportional to the scanning speed is obtained by the laser beam detector 19. Through the function of the PLL frequency multiplier circuit described above, a grid clock signal F which has a frequency multiplied from the pulse signal S only by that amount of the set multiplication ratio and which is proportional to the scanning speed, is obtained from the voltage-controlled oscillator VCO.
This grid clock signal F, as shown in FIG. 7, is used in the following process: the image signal is read by the photoelectric transducer 12, undergoes A/D conversion in a converter 20, and is recorded into the image signal memory 21; the image signal is read out again, undergoes image processing in a component 22, and is recorded into the processed image signal memory 23; the resulting output signal is then read out and fed to the light modulator 14 FIG. 4.
Because this type of image reproduction apparatus is used in the fields of printing or plate-making and high-grade image reproduction or copying, the necessary image processor 22 includes tone correction, image enhancement, making halftones for making printing plates, enlarging, reducing, and so on. The multiplication ratio of the PLL frequency multiplier circuit which provides the grid clock signal F is determined by the frequency of the pulse signals S and the desired image resolution. For example, for a pitch of 100 .mu.m for the light shield slits in the grid plate 18 and a resolution of 4 .mu.m, the multiplication ratio should be 25. Of course, if it is possible to fabricate a grid plate 18 having the same light shield slit pitch as the needed resolution, the PLL frequency multiplier circuit becomes unnecessary, and the pulse signal S can simply be used as the grid clock signal F.
Although the performance of image reproduction apparatus using the technology described above has been improved somewhat by the means just described, the grid clock signal F which controls the various processing is obtained on the assumption that the operation of the galvano mirrors 6 and 6' is extremely stable. However, in actuality, the operation of these galvano mirrors is sometimes not stable, thus causing a loss of stability in the grid signal itself. This instability becomes more noticeable the higher the resolution required and the higher the magnification ratio of the grid clock signal.
In addition, if the grid clock signal is obtained by using the galvano mirrors, because only the outward swing, for example, of the mirrors is used, the time for the return swing is not used. Thus, for processing using this type of grid clock signal, there is a time with no signal corresponding to the return swing of the mirrors, which is approximately 40% of each cycle time. Because the various image processing must be performed in the remaining 60% of the cycle time, high-speed processing is necessary, and errors are more likely to occur when the various image processing is performed at high speed. As a result, an excessive amount of precision and features are required by the electrical circuits, thus increasing the complexity and cost of the circuits, and further increasing the possibility of errors.
In this way, with an image reproduction apparatus using conventional galvano mirrors, when laser scanning is used to generate the grid clock signal and perform the various image processing, because the grid clock signal itself lacks stability, the apparatus is insufficient for high-quality image reproduction, and the various problems discussed herein occur.
It is a general object of this invention to solve the above-described problems of the image reproduction apparatuses, which reproduce images by deflecting a laser beam through the use of galvano mirrors, and wherein various image processing is performed using a grid clock signal.